The present invention relates to offshore drilling structures and particularly to a method for strengthening and consolidating the sea bottom formations that underlie a gravity-anchored offshore structure. The term "gravity-anchored" structure is used to describe the steel or concrete structures that have been proposed for use in the shallow waters offshore Northern Alaska or Canada and the similar type structures that have been used in the North Sea. All of these structures are massive and depend upon their weight, skirt configuration (if any), and the strength of the underlying soil for anchoring them in position at the offshore location.
In most locations where such structures are used the bottom underlying the structure comprises an unconsolidated soil formation. Often foundation skirts are used to key the base of the structure into the soil. In order to minimize the base area required, a means must be provided for strengthening and consolidating the bottom to support the structure and prevent the structure shifting as a result of wave action or, in the case of Arctic structures, moving ice contacting the structure.
In addition to the above problems, it is also possible for the soil itself to become liquified as a result of the differential pore pressure between the external hydrostatic pressure and the pressure existing in the soil underlying the structure. In these conditions, the soil will liquify and the ability of the sea bottom formation to support the structure will be greatly reduced.
The above conditions have been recognized in the prior art and various steps have been taken to provide a solution. One of the simplest approaches is to replace the soil formation where the structure is to be located with other material such as gravel and the like to provide the necessary foundation for the structure. While this is a solution, it is expensive and at times difficult to accomplish.
Other solutions have been proposed such as attempting to equalize the pore pressure in the formation underlying the structure with the external hydrostatic pressure. This approach is described in U.S. Pat. No. 3,962,878.
Additional approaches have been to modify the portion of the structure that is resting on the bottom so that the design itself will resist movement of the structure. These approaches are described in U.S. Pat. Nos. 2,895,301 and 4,037,424. While these approaches improve the anchoring of the structure in the desired location, they do not address the problem of supporting the structure on the soft soil and silt which forms the sea bottom in many offshore locations.
An additional approach to stabilizing and strengthening soil and rock formations in underground structures is described in U.S. Pat. No. 5,026,215. This patent teaches injecting a cement slurry into the formation to strengthen and consolidate the formation. The slurry uses water and a combination of particulate slag and Portland cement to form the cementitious material. In particular, the slag is an ultra fine material having a surface area of greater than 9500 cm.sup.2 /g. The patent utilizes fresh water as the aqueous phase in order to be compatible with the Portland cement, and it is therefore not suitable for use in offshore locations where the water is salt water and incompatible with Portland cement.
A further approach to strengthening weak soils is discussed in a paper entitled "Deep Cement Mixing Experience in Japan", G. Dennis et al., presented at the Seabed Strengthening Symposium at Denver, Col., Jun. 4, 1985. This paper describes the mixing in place of Portland cement with weak soils to strengthen them. The paper also describes various types of machinery that can be used to perform in-place mixing of Portland cement and soil.